The present invention relates to a method for forming a ceramic coating on a part by irradiating it with a laser beam.
Ceramic coatings have been conventionally formed on a substrate by use of a gas phase method, a solid phase method or a liquid phase method. The gas phase method comprises once vaporizing or ionizing materials capable of forming ceramic coating, and then depositing the materials on the substrate to form the ceramic coating. According to the gas phase method, there can be prepared any coating of various compounds and the thus prepared coating is very pure and of good properties. It also has such advantages as good adhesion to the substrate. However, since it is necessary to maintain the reaction temperature over 1000.degree. C. in the gas phase method, this method is usable only for substrates having a high temperature resistance. This method also has a disadvantage in that it requires an apparatus of large scale because this method should be carried out under a vacuum. The solid phase method comprises adhering certain materials to the substrate and then reacting the materials with the substrate to form a coating. Since the reaction of the materials with the substrate is used for forming the coating, the method has disadvantages in that the reaction temperature should be maintained high and the kind of ceramic coating is limited to specific ones.
On the other hand, although various liquid phase methods have been tried, the spray method is the most popular one. This method comprises preparing a metal alkoxide or an organic metal compound in a liquid state, coating it onto the substrate and heat-decomposing the compounds to form a coating on the substrate. By this method, the formation of the coating is relatively easy, but there are many disadvantages such as that the thus formed coating is not dense, it is difficult to control the thickness of the coating, adhesion between the substrate and the coating is not sufficient, and it is necessary to heat-treat each substrate. Examples of methods other than the spray method for forming a ceramic coating on a metal substrate include an anodizing process. According to this anodizing process, a dense ceramic coating is formed, but this method has a disadvantage in that the formed ceramic is limited to Al.sub.2 O.sub.3. Furthermore, an anode spark discharge method is known as a method utilizing electrolysis. For example, Japanese patent publication (hereinafter referred to as J.P. KOKOKU) Nos. 58-17278, 59-28386, 59-28638 and 59-45722 disclose methods for forming a ceramic coating on a substrate, which comprise conducting an electrolysis in an aqueous alkali solution of silicate or various metal salts of oxygen acid and generating spark discharge between anode metal, silicic acid ions and metal oxygen acid ions present near the anode. Although various ceramic coatings can be formed on a substrate by these methods, there are disadvantages in that the thus formed coatings are porous and the substrates to be used are limited to metal, in particular, Al, Mg, Ti and the like.
Therefore, according to the conventional methods, except for some liquid phase methods, heating at a high temperature is required, so that a ceramic coating can be formed only on certain substrates which have superior heat resistance. In addition, according to the conventional method, it is very difficult to partially form the ceramic coating on the substrate.